The Third Generation Partnership Project (3GPP) Technical Report (TR) 25.814 V7.0.0, Physical Layer Aspects for Evolved Universal Terrestrial Radio Access (UTRA) (Release 7), June 2006, and 3GPP Technical Specification (TS) 36.211 V8.1.0, Physical Channels and Modulation (Release 8), November 2007, describe the physical layer of an evolved UTRA network (E-UTRAN) able to operate over a very wide range of operating channel bandwidths and carrier frequencies and with small-diameter “micro” cells up to large-diameter “macro” cells having 100-km cell ranges. 3GPP promulgates specifications that standardize many kinds of cellular wireless communication systems.
FIG. 1 depicts a typical cellular wireless telecommunication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a mobile station (MS), or remote terminal or user equipment (UE), via the appropriate base station(s) (BSs), which communicate with each other through downlink (DL) (i.e., base-to-mobile or forward) and UL (i.e., mobile-to-base or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or Node B in 3G vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 may be modified in an E-UTRAN and other communication networks. For example, the functionality of the RNCs 12, 14 may be moved to the node Bs 22, 24, 26, and other functionalities may be moved to other nodes in the network.
In order to handle the different radio conditions that may occur in an E-UTRAN, orthogonal frequency division multiplexing (OFDM), or orthogonal frequency division multiple access (OFDMA), is used in the downlink. OFDMA is a radio access technology (RAT) that can adapt to the different radio propagation conditions that can occur in an E-UTRAN. In particular, an OFDM system can adapt its DL transmission parameters not only in the time domain, as in current communication systems, but also in the frequency domain. OFDMA communication systems are also described in the literature, for example, U.S. patent application Ser. No. 11/289,184 by B. Lindoff et al.
In an OFDMA communication system, the available data stream is portioned into a number of narrowband subcarriers that are transmitted in parallel. Because each subcarrier is narrowband, each carrier experiences only flat-fading, which makes it easy to demodulate each subcarrier. A basic time-frequency structure of a DL in an OFDM system is depicted in FIG. 2, which shows a plurality of OFDM sub-carriers that are contiguous in the frequency direction. The radio resource devoted to a particular user may be called a “block” or a “chunk”, which is a particular number of particular sub-carriers used for a particular period of time. Different groups of sub-carriers are used at different times for different users, and FIG. 2 illustrates resource blocks for four users A, B, C, D. In the downlink of the exemplary OFDM system depicted by FIG. 2, a block includes 12 sub-carriers (not all of which are shown, for clarity) spaced apart by 15 kilohertz (kHz), which together occupy approximately 180 kHz in frequency, and 1.0 millisecond (ms) in time. It will be understood that the arrangement of FIG. 2 is just an example and that other arrangements can be used.
For receiver synchronization and other purposes, reference symbols or signals, which may be called pilots, can be transmitted from each base station at known frequency and time instants. Such reference signals are described for example in Section 7.1.1.2.2 of 3GPP TR 25.814 and Sections 6.10 and 6.11 of 3GPP TS 36.211. An exemplary time-frequency structure with eight such pilots 302 is depicted in FIG. 3, which shows eight sub-carriers having the pilots 302 in the OFDM time-frequency plane. Other OFDM sub-carriers 304 transport data, but for clarity these are indicated in FIG. 3 at only one instant in the time-frequency plane. It will be understood that each resource block typically includes a few pilots on different sub-carriers. It will also be understood that a BS may use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas may send respective, different pilots.
According to Section 7.1.2.4 of 3GPP TR 25.814 and Sections 6.11 and 5.7 of 3GPP TS 36.211, an E-UTRAN has initial access channels, such as a Synchronization Signal (SyS) and a random-access channel (RACH), that are robust, enabling a UE to access the system under many different radio conditions. The SyS in E-UTRAN consists of a Primary Synchronization Signal (P-SyS) and a Secondary Synchronization Signal (S-SyS). Three P-SyS are currently defined in E-UTRAN and are distributed over the E-UTRAN cells. Comparable synchronization and random-access channels are often provided in other digital communication systems, although they may be given different names.
In order to access the network, a UE carries out a cell-search algorithm that starts with the UE's correlating its received signal with its local replicas of all three P-SyS to synchronize itself with the system timing. After this step, the UE knows the position of the S-SyS and proceeds to a second stage of the cell-search algorithm, in which the UE decodes the S-SyS, which contains the cell's group identification (ID). The cell's group ID, together with the information about which of the three P-SyS is present, establishes the physical-layer cell ID of the cell. The UE then has all the information it needs to read broadcast system information and establish communication with the network.
The P-SyS in E-UTRAN are based on Zadoff-Chu (ZC) sequences, which are a special class of generalized chirp-like (GCL) sequences. A ZC sequence having a length N, where N is odd, and a sequence index u is defined by the following expression:Zu(k)=exp(−j·π/N·u·k·(k+1)),k=0,1, . . . ,N−1.The three different P-SyS signals in E-UTRAN are ZC sequences of the same length N with different sequence indices u.
ZC sequences have special properties, some of which are desirable but some which need special attention. On the positive side, ZC sequences belong to the class of CAZAC sequences, which have Constant Amplitude (CA), constant magnitude cross-correlations across all lags and a Zero Auto-Correlation (ZAC) for lags not equal to zero. These properties make ZC sequences very attractive for synchronization applications. On the negative side, the auto-correlation behavior of ZC sequences in the presence of frequency offsets needs to be considered because it produces multiple peaks in the auto-correlation signal that interfere with accurate synchronization.
With multiple peaks in the autocorrelation signal, it can be difficult for a UE to decide which peak is the correct one and achieve proper synchronization. If extra receiver components or functions are implemented to identify the correct autocorrelation peak, several disadvantages are introduced. The receiver is made more complex, and the area of a semiconductor chip required for such processing as well as the amount of electrical energy consumed by such processing are increased. The greater energy consumption decreases the life time of battery-powered UEs, such as mobile phones, pagers, etc.